The invention relates to a method for automated measurement of the residual magnetic field strength of magnetized ferromagnetic workpieces, in particular of pipes made of steel.
Fault tests of steel pipes are frequently performed with magnetic methods or with magneto-inductive methods by using a magnetic bias. The test piece has then residual magnetic fields of a strength and direction that depends on the exact implementation of the test and the properties of the material. However, an undesired magnetization can also occur during pipe manufacture, during further processing and when the pipes are transported.
Residual magnetic fields can be troublesome for the further processing at the customer site and should therefore be kept as small as possible.
These residual magnetic fields are very troublesome particularly in the vicinity of the pipe ends, because the arc generated when the pipes are welded together is adversely affected by these residual magnetic fields. If the residual magnetic fields at the end face of the pipe ends are too strong, the arc is deflected, making welding difficult or even impossible.
For the aforementioned reasons, demands from customer may make it necessary to demagnetize pipe ends having a residual magnetic strength that should actually still be tolerable.
To test if the residual magnetic fields have dropped below a predetermined value, the residual magnetic fields must be measured on the pipe.
Typically, the residual magnetic fields are hereby measured at the end face of the pipe using handheld measuring devices, for example Hall probes. The use of devices for measuring magnetic fields (Gauss meter) is fundamentally known, for example, from OS 2 238 012 and is frequently used to determine the residual magnetic field on pipes. The residual magnetic field is measured by placing a calibrated Hall probe on the end face of the pipe end.
This measurement process has the disadvantage that the measurement on the end face is quite difficult to automate at reasonable costs due to structural reasons.
Sometimes, freestanding magnetic field probes are used for automatic measurements of pipes moving past the probes. However, these measurements are not very informative because a contacting measurement of the pipes would actually be required which, however, is typically omitted due to the high risk of damage by the passing pipe.
Moreover, a precisely defined distance to the end face of the pipe is very difficult to realize with moving pipes. Variations of the signals can here not be unambiguously associated with changes in the residual field strength, because these variations could also be caused by an unintentional change of the spacing between the probe and the end face of the pipe. The signal strength decreases significantly with increasing distance, which makes evaluation of the measurement results much more difficult.